The invention relates to vesicular imaging. More particularly, this invention relates to methods for forming vesicular images and to the use of these images as masks for imaging printing plate precursors, especially flexographic printing plate precursors.
Flexographic printing plates are used in letterpress printing, particularly on surfaces which are soft and easily deformable, such as packaging materials, e.g., cardboard, paper, and plastic packaging films. Flexographic printing plates can be prepared from imageable elements that comprise a photosensitive layer on a substrate. The element is imaged with ultraviolet and/or visible radiation and then developed with a suitable developer leaving a printing relief, which can be used for flexographic printing. If after exposure to radiation the exposed regions of the photosensitive layer are removed in the developing process, the element is positive working. Conversely, if the developing process removes the unexposed regions and the exposed regions remain, the element is negative working.
Imaging of the imageable element with ultraviolet and/or visible radiation is typically carried out through a mask, which has clear and opaque regions. Imaging takes place in the regions of the photosensitive layer under the clear regions of the mask but does not occur in the regions of the photosensitive layer under the opaque regions of the mask. The mask is usually a photographic negative of the desired image. If corrections are needed in the final image, a new mask must be made. This is a time-consuming process. In addition, the mask may change slightly in dimension due to changes in temperature and humidity. Thus, the same mask, when used at different times or in different environments, may give different results and could cause registration problems.
Direct digital imaging of printing plate precursors, which obviates the need for exposure through a mask, is becoming increasingly important in the printing industry. In these processes, a computer controlled laser scans and images the photosensitive layer of the printing plate precursor. However, it has not been practical to use lasers to image flexographic printing plate precursors. These elements have low photosensitivity and require long exposure times even with high-powered lasers. In addition, most of the imageable materials used in these elements have their greatest sensitivity in the ultraviolet region of the spectrum. Although ultraviolet emitting lasers are known, economical and reliable ultraviolet lasers with high power are not readily available. However, relatively inexpensive infrared lasers that have a useful power output are readily available. These lasers have been used to form a mask over the photosensitive layer.
Flexographic printing plate precursors that comprise an imageable layer that is ablatable by infrared radiation over the photosensitive layer have been used to retain the advantages of direct digital imaging. The imageable layer is imaged with an infrared laser to form a mask and the resulting element exposed with ultraviolet and/or visible radiation through the mask. However, because ablation produces debris, the platesetter used to image ablative masks requires additional filtration systems to prevent the debris from contaminating the optics of the platesetter. In addition, some of the ablatable layers require large amounts of expensive infrared absorbers.
Thus, a need exists for imageable elements useful as flexographic printing plate precursors that have the advantages of direct digital imaging but do not have the disadvantages of laser ablation.
In one aspect, the invention is a method for forming vesicular images. The method comprises the steps of:
(A) providing an imageable element comprising an imageable layer on a substrate, in which the imageable layer comprises a thermally imageable vesicular imaging composition; and
(B) thermally imaging the imageable layer and forming an imaged imageable layer, the imaged imageable layer comprising a vesicular image;
in which the thermally imageable vesicular imaging composition comprises a sensitizer and polymeric material.
In one aspect, the method additionally comprises a thermal development step. In another aspect, the imageable element is overall (flood) exposed with ultraviolet radiation after step (A) and before step (B).
In another aspect, the invention is a method for forming a relief image, the method comprising the steps of:
(A) providing an imageable element, the imageable element comprising, in order:
(i) a substrate;
(ii) a photosensitive layer, the photosensitive layer comprising a photoimageable composition, and
(iii) a masking layer, the masking layer comprising a thermally imageable vesicular imaging composition, the thermally imageable vesicular imaging composition comprising a sensitizer and a polymeric material;
(B) thermally imaging the masking layer and forming a vesicular image in the masking layer;
(C) overall exposing the imageable element to actinic radiation through the masking layer and forming an imaged imageable element comprising imaged and complementary unimaged regions in the photosensitive layer; and
(D) developing the imaged imageable element in a developer and forming the relief image by removing either the unimaged regions or the imaged regions.
In another aspect, the substrate is a flexible substrate and the photosensitive layer comprises a negative working photosensitive composition. The resulting relief image is useful as a flexographic printing plate.